MOS field-effect transistors with very thin silicon dioxide based gate dielectrics may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high-k dielectric materials can reduce gate leakage. To ensure acceptable transistor performance, it may be necessary to form a transition oxide between the underlying substrate (e.g., a silicon wafer) and the high-k dielectric layer. If, however, there is an abrupt dielectric constant transition between the interfacial oxide and the high-k dielectric, the resulting film may be unreliable. A transistor with such a film may have an unstable threshold voltage (Vt) if the transition oxide breaks down quickly, when subjected to an applied field.
Accordingly, there is a need for an improved process for making a semiconductor device that includes a high-k gate dielectric. There is a need for a process for forming a gate dielectric that does not show a sharp dielectric constant transition between a high-k dielectric and an interfacial oxide. The method of the present invention provides such a process.
Features shown in these figures are not intended to be drawn to scale.